The invention relates to a method for fabricating memory cells in accordance with the stacked principle, in which a bonding layer immediately above the plug is formed between a lower capacitor electrode of a ferroelectric storage capacitor and a conductive plug of polysilicon which is formed beneath the ferroelectric storage capacitor and is used to electrically connect the capacitor electrode to a transistor electrode of a selection transistor formed in or on a semiconductor wafer. An oxygen diffusion barrier is formed above the bonding layer, and after the ferroelectric has been deposited it is subjected to a rapid thermal processing step in an oxygen atmosphere after the formation of the ferroelectric and before a ferro anneal thereof.
In the case of ferroelectric memory cells constructed according to the stacked cell principle, it is typical for transistors to be fabricated in or on a semiconductor wafer. Then, an intermediate oxide is deposited. The ferroelectric capacitor modules are fabricated on the intermediate oxide. The ferroelectric capacitor modules are connected to the transistors by a plug, which in the case of the stacked cell principle is located directly beneath the capacitor module.
To condition the ferroelectric layer of the ferroelectric capacitor module, it is necessary to carry out a ferro anneal in an oxygen atmosphere at temperatures of up to 800xc2x0 C. During the ferro anneal, the plug, which generally is formed of polysilicon or tungsten, has to be protected from oxidation, since otherwise the electrical connection between the lower capacitor electrode and the transistor is irreversibly broken. Moreover, reactions between the electrodes, the ferroelectric and the plug is to be avoided wherever they adversely affect the functionality of the chip.
All products that are currently commercially available with ferroelectric layers are constructed according to the offset cell principle and have an integration density of only a few kilobits up to one megabit.
To protect the plug from oxidation in a ferroelectric memory constructed according to the stacked cell principle, layer systems that contain an oxygen diffusion barrier and a bonding layer below have been introduced. However, it is very difficult to prevent the oxidation of the oxygen diffusion barrier and in particular of the bonding layer below and of the plug of polysilicon or tungsten or its surface from the side during the ferro anneal.
Experiments carried out by the inventors on prototypes have shown that competing processes were taking place in the bonding layer consisting of titanium during the ferro anneal.
FIG. 1 shows a diagrammatic cross section through part of a ferroelectric memory cell constructed in accordance with the stacked cell principle. FIG. 1 shows a plug 1, which leads through an intermediate oxide layer 7 (TEOS) and is made, for example, from polysilicon, a lower part 2 of a bonding layer, for example made from TiSi2, located immediately above it, an upper part 3 of the bonding layer, for example made from Ti, located above the lower part of the bonding layer, a lower part 4 of an oxygen diffusion barrier, for example made from Ir, and above this a second part 5 of the oxygen diffusion barrier, for example made from IrO2. Above the upper IrO2 section 5 of the oxygen diffusion barrier is the lower capacitor electrode 6, for example consisting of Pt. The oxidation which takes place from the side during the ferro anneal is indicated by bold arrows filled in black in FIG. 1, and the simultaneous formation of TiSixe2x80x94Ir in the bonding layer 2, 3 is denoted by unfilled arrows in FIG. 1. A circle denoted by II surrounds an excerpt, details of which are illustrated in FIGS. 2A and 2B, to which the description below relates. The processes and formations diagrammatically depicted in FIGS. 2A and 2B result from transmission electron microscope (TEM) images produced by the inventors. FIG. 2A once again uses a bold arrow to show the oxidation of the bonding layer 2, 3 which takes place from the side. In this case, an insulating TiSixe2x80x94O region 10 forms from the side. The arrows that are not colored in in black illustrate the siliciding, namely the formation of TiSixe2x80x94Ir, which takes place from above and below. In FIG. 2A, the conductive TiSixe2x80x94Ir layer has taken place more quickly than the formation of the insulating TiSixe2x80x94O layer 10 from the side.
By contrast, in FIG. 2B the insulating TiSixe2x80x94O region 10 from the side has formed over the entire width of the plug 1, and the latter is no longer electrically connected to the lower electrode 6 of the ferroelectric capacitor.
It is therefore found that, despite the Ir/IrOx coverage of the Ti bonding layer, there is an oxygen diffusion path along the IrOx/TEOS interface, which can partly oxidize the bonding layer 2.
During the experiments carried out by the inventors, it has emerged that the rate at which the formation of TiSixe2x80x94O proceeds from the side and at which the simultaneous formation of TiSixe2x80x94Ir from above and below takes place are dependent on the temperature at which these reactions occur.
With the aid of a rapid thermal processing (RTP) step in oxygen, it is possible to accelerate the formation of TiSixe2x80x94Ir from above and below in the bonding layer compared to the formation of TiSixe2x80x94O from the side.
A reference by M. Heintze, A. Catana, P. E. Schmid, F. Lxc3xa9vy, P. Stadelmann and P. Weiss, titled xe2x80x9cOxygen Impurity Effects On The Formation Of Thin Titanium Silicide Films By Rapid Thermal Annealingxe2x80x9d, J. Phys. D: Appl. Phys., Bd. 23, 1990, pages 1076-1081, XP001124373, deals with the behavior of oxygen impurities in the diffusion between titanium and silicon during an RTP step in a temperature range from 480xc2x0 C. to 800xc2x0 C. By way of example, FIG. 1 of the document shows concentration profiles of sintered Ti/Si diffusion pairs in the case of an RTP step at 500xc2x0 C. for 20 seconds, 60 seconds and 120 seconds. Furthermore, FIG. 2 of that document shows concentration profiles of titanium silicide specimens that have been treated by an RTP step for 60 seconds at 550xc2x0 C., 600xc2x0 C. and 800xc2x0 C. On the basis of their tests, the editors come to the conclusion that the oxygen prevents the complete reaction of the siliciding of titanium at temperatures below 650xc2x0 C. By contrast, above this temperature it is observed that titanium is completely consumed by the Ti/Si reaction and that the silicide that results is substantially free of oxygen contamination (FIG. 2C). The discoveries obtained in this document through tests therefore concur with the findings described by the present inventors in the introductory part of the description of the present patent application on the basis of FIGS. 1, 2A and 2B that the rate at which the formation of TiSixe2x80x94O and the simultaneous formation of TiSixe2x80x94Ir take place is dependent on the temperature at which these reactions take place, and that with the aid of an RTP step at high temperature it is possible to accelerate the formation of TiSixe2x80x94Ir compared to the formation of oxide. However, the editors of the above-mentioned document do not give any indication whatsoever of calculating an optimum temperature range, and apart from the detail that the thickness of the Ti/Si specimen is 40 nm, their explanations do not give any statements concerning the influencing of the siliciding reaction as a function of the dimensions of a component.
Virtually identical results are obtained in the reference by Wee A T S, Huan A C H, Thian W H, Tan K L, Hogan R, titled xe2x80x9cInvestigation of Titanium Silicide formation using secondary Ion mass spectrometryxe2x80x9d, Mat. Res. Soc. Symp. Proc., Vol. 342, 1994. The rapid high-temperature treatment referred to in that document as RTA (rapid thermal annealing) takes place for twenty seconds in a nitrogen atmosphere at approximately 650xc2x0 C. Therefore, the rapid high-temperature treatment of that document differs from the RTP treatment in the present patent application, which takes place in an oxygen atmosphere. FIG. 1B of that document shows that the titanium siliciding dominates the oxidation of titanium during an annealing step at 620xc2x0 C. (100 minutes). The curves shown in FIG. 2 of this document deal with the results which are achieved by the abovementioned RTA step in a nitrogen atmosphere, and in particular FIG. 2A, in accordance with the editors"" information, proves that the RTA step in a nitrogen atmosphere allows the siliciding of titanium to take place substantially uninfluenced by oxygen diffusion. This document too fails to describe any calculation algorithm with which it is possible to calculate the optimum temperature range for an RTP step in an oxygen atmosphere.
International Publication WO 00/39842 A discloses a ferroelectric memory cell constructed in accordance with the stacked principle as mentioned in the introductory part of the description. The third paragraph of page 15 of document D1 describes a high-temperature annealing step (for example a ferro anneal) in an oxygen-containing atmosphere in order to crystallize out the dielectric metal oxide-containing layer, i.e. the ferroelectric, this ferro anneal, in the case where SBT is used for the ferroelectric, being carried out for one hour at 800xc2x0 C.
Document D1 does not at any point mention an RTP step in an oxygen atmosphere being carried out prior to the ferro anneal.
U.S. Pat. No. 5,932,907 has likewise recognized the problem of oxidation of a precious metal layer which occurs during the ferro anneal competing with the siliciding of this layer, and proposes an oxygen diffusion barrier between unreacted precious metal and a precious metal silicide layer. This document does not give any indication of an RTP step in an oxygen atmosphere being carried out prior to a ferro anneal.
It is accordingly an object of the invention to provide a method for fabricating ferroelectric memory cells that overcomes the above-mentioned disadvantages of the prior art methods of this general type, which are constructed in accordance with the stacked principle in such a way that for a predetermined thickness of the Ti bonding layer it is possible to find an associated ideal RTP temperature at which the layer system remains conductive. With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating ferroelectric memory cells in accordance with the stacked principle. The method includes providing a conductive plug formed of polysilicon, and forming a bonding layer directly above the conductive plug and between a lower capacitor electrode of a ferroelectric storage capacitor and the conductive plug. The conductive plug is formed beneath the ferroelectric storage capacitor and electrically connects the lower capacitor electrode to a transistor electrode of a selection transistor formed in or on a semiconductor wafer. An oxygen diffusion barrier is formed above the bonding layer. A rapid thermal process (RTP) step in an oxygen atmosphere is performed, after a ferroelectric deposition and before a ferro anneal has been formed. The RTP step includes:
determining an oxygen rate of the bonding layer and a first diffusion coefficient (Doxygen) of oxygen in a material of the bonding layer in dependence on temperature;
determining a second diffusion coefficient (Dsilicon) of silicon in the material of the bonding layer in dependence on the temperature; and calculating an optimum temperature range for the RTP step from the first and second diffusion coefficients (Doxygen and Dsilicon) for a predetermined layer thickness and a layer width of a layer system formed of the bonding layer and the oxygen diffusion barrier from the relationship                     (                  d          BARR                )            2              D      silicon        ⁢      ⟨                            (                      b            BARR                    )                2                    D        oxygen            
in which the left-hand term denotes a period of time required for full siliciding of the bonding layer, and the right-hand term denotes a period of time required to fully oxidize the bonding layer,
dBARR denotes the predetermined layer thickness of the layer system formed of the bonding layer and the oxygen diffusion barrier,
bBARR denotes half the layer width of the layer system containing the bonding layer and the oxygen diffusion barrier, so that during the RTP step siliciding of the bonding layer takes place more quickly than its oxidation.
In the invention, the oxidation rate of the bonding layer is determined, and from this the diffusion coefficient of oxygen in the material of the bonding layer, for example titanium, is determined as a function of the temperature. The rate at which a TiSixe2x80x94Ir layer is formed from a titanium layer is formed, and from this the diffusion coefficient as a function of the temperature is also determined. Then, given a predetermined titanium layer thickness, it is possible to use the temperature-dependent diffusion coefficient and the oxidation rate to calculate the optimum temperature which is required to enable the TiSixe2x80x94Ir formation to take place quickly enough, i.e. more quickly than the simultaneous formation of the insulating TiSixe2x80x94O regions, in order to maintain the conductivity of the layer system.
The invention gives a formula that can be used to calculate the optimum temperature range or the optimum temperature for the RTP step:                                                         (                              d                BARR                            )                        2                                D            silicon                          ⁢                  ⟨                                                                      (                                      b                    BARR                                    )                                2                                            D                oxygen                                      ,                                              (        1        )            
In relationship (1), the left-hand term denotes the period of time required for full siliciding of the bonding layer, and the right-hand term denotes the period of time required to fully oxidize the bonding layer;
dBARR denotes the layer thickness of the system containing the oxygen diffusion barrier and the bonding layer thereof;
bBARR denotes half the layer width;
Dsilicon denotes the temperature-dependent diffusion coefficient of silicon; and
Doxygen denotes the temperature-dependent diffusion coefficient of oxygen in the material of the bonding layer.
In accordance with an added mode of the invention, there is the step of forming the bonding layer, due to the RTP step, to contain a lower layer made from TiSi2 and an upper layer made from Ti and disposed directly above the lower layer.
In accordance with a further mode of the invention, there is the step of forming the oxygen diffusion barrier, prior to the RTP step, to contain a diffusion lower layer made of Ir, which lies directly above the upper layer of the bonding layer, and a diffusion upper layer of IrO2 directly covering the diffusion lower layer of the oxygen diffusion barrier.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating ferroelectric memory cells, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.